1. Field of the Invention
This invention relates to antipilferage systems, particularly to such systems as utilize a marker containing a ferromagnetic material which is detected by subjecting the material to an alternating magnetic field and sensing the perturbations resulting from reversals of the magnetization of the material.
2. Description of the Prior Art
Antipilferage systems such as are useful in the remote detection of objects such as retail merchandise and library books as they pass through an interrogation zone are now well known. Perhaps beginning with a 1934 French Patent (French Patent No. 763,681, issued to Pierre Arthur Picard) and continuing through a number of recent U.S. patents, such as U.S. Pat. Nos. 3,665,449 (Elder and Wright) and 3,790,945 (Fearon), it is evident that investigators have appreciated that the presence of a marker may be uniquely distinguished from other even relatively common articles if one provides as a part of the marker a section of a high permeability ferromagnetic material. It has been appreciated that when the magnetization of such a material is reversed by a periodically alternating magnetic field such as provided in an interrogation zone through which the marker is passed, the magnetization reversal would result in the production of harmonics of the alternating field. The detection of the harmonics could then be used to indicate the presence of an object to which the marker is affixed.
More particularly, the recent U.S. patents reflect the discovery that if care is given to the shape of the ferromagnetic material, i.e., that if an "open-strip" is provided, very high order harmonics will be generated in a rapidly varying magnetic field. By "open-strip" is meant a section of material which when magnetized has separate poles, i.e., a strip which is not closed or wound upon itself, and which preferably has a ratio of length to square root of cross sectional area of at least 150. Such a discovery has resulted in the development of an antipilferage system such as disclosed in Elder et al which has enjoyed considerable commercial success, but which has heretofore been limited to protecting objects shaped such that a linearly extending marker approximately 7 inches (18 cm) long could be affixed thereto. Correspondingly, such a system has not heretofore been useful with objects such as microfilm cartridges and magnetic recording cassettes in which the objects did not extend over such linear dimensions. The need to also protect such objects makes it desirable to have a shorter marker which would produce the same response in the interrogating field provided in previously installed systems. The French Patent referenced hereinabove suggests that a metal piece employing polar extensions, such as are shown in FIG. 7 of that reference may be provided to increase the sensitivity of the apparatus. However, there is no indication in that specification of any appreciation of any manner by which very high order harmonics could be generated. Rather, that reference indicates that the size of the metal body used as the marker varies only the amplitude of the received wave, not the order of the harmonic. Such increases in size, be they shaped as polar extensions or whatever, do not suggest a capability of protecting small objects.